ST_SimplifyVW

Signature

GEOMETRY ST_SimplifyVW(GEOMETRY geom, DOUBLE distance);

Description

Simplifies geom using the Visvalingam-Whyatt algorithm with a distance tolerance (distance).

Ensures that resulting polygonal geometries are valid.

Note

The Visvalingam-Whyatt algorithm does not preserve topology. To preserve the topology, use ST_SimplifyPreserveTopology. But note that ST_SimplifyVW is significantly faster.

Examples

With LINESTRING

SELECT ST_SimplifyVW('LINESTRING(1 1, 3 2, 1 2, 2 3, 1 4, 4 3, 4 1)', 1.5);

-- Answer: LINESTRING (1 1, 2 3, 4 3, 4 1)


SELECT ST_SimplifyVW('LINESTRING(1 1, 3 2, 1 2, 2 3, 1 4, 4 3, 4 1)', 2);

-- Answer: LINESTRING (1 1, 4 3, 4 1)

With POLYGON

SELECT ST_SimplifyVW('POLYGON((2 1, 1 2, 2 2, 2 3, 3 3, 3 2, 4 2, 4 1, 3 0, 2 0, 2 1))', 0.5);

-- Answer: POLYGON ((2 1, 2 3, 4 2, 3 0, 2 1))

With MULTIPOLYGON

SELECT ST_SimplifyVW('MULTIPOLYGON(((2 1, 1 2, 2 2, 2 3, 3 3, 3 2, 4 2, 4 1, 3 0, 2 0, 2 1)), 
                                   ((1 3, 0 4, 3 4, 1 3.5, 1 3)))', 0.5);

-- Answer: MULTIPOLYGON (((2 1, 2 3, 4 2, 3 0, 2 1)), ((1 3, 0 4, 3 4, 1 3)))

With a large distance

If the distance tolerance is too large (compared to the geometry), the Geometry may be oversimplified:

SELECT ST_SimplifyVW('POLYGON((2 1, 1 2, 2 2, 2 3, 3 3, 3 2, 4 2, 4 1, 3 0, 2 0, 2 1))', 2);

-- Answer: GEOMETRYCOLLECTION EMPTY

With (MULTI)POINT

POINTs and MULTIPOINTs cannot be further simplified:

SELECT ST_SimplifyVW('POINT(190 300)', 1);

-- Answer: Error

See also